The use with reference to New England and the Maritime Provinces of Canada perhaps reflects an origin as a nautical usage (perhaps originally alluding to these regions as being down wind: see down wind at windn.1 19). However, perhaps compare also down Northadv. and discussion at that entry, and also down countryadv.

1. New England (esp. eastern Maine) and the Maritime Provinces of Canada; spec. the eastern coastal parts of this region.Chiefly used within New England and the Maritime Provinces to denote coastal parts of the region located further east. The idea that nowhere do residents identify their own part of the region as ‘down east’ has formed the basis of many humorous stories and articles (cf. quot. 1906).

1817 Exeter
(New Hampsh.)
Watchman 7 May 3/1
We talk of the ‘Westward’, as we once did of the ‘down east’, and with just as much distinctness and discrimination.

1887 E. IngersollDown East Latch Strings ix. 77
Moosehead lake is the largest of the hundreds of inland seas with which ‘down east’ is provided.

1906 Pine Tree Mag. Oct. 208/2
I looked across the Bay of Fundy and felt thankful that I would not have to cross its waters to reach Down East.

1999 E. BrechlinMaine Adventure Guide 249
The real Down East does not begin until you cross over to the eastern side of the Penobscot River.

2. The eastern parts of the United States or the central parts of Canada (esp. Ontario) as regarded by inhabitants of more western parts. Cf. back Eastn.

1849 Wisconsin Farmer, & Northwestern Cultivator Aug. 173/1
Who of our Wisconsinites does not love fruit?..from the littlest bub or sis..up to the grandparent who spent some of the happiest moments of child-hood among the fruitful orchards..of ‘Down East’.

1873 Literary World May 188/3
Mr. Hayne did not look beyond Mason and Dixon's line..when he introduced into a biographical sketch of a luckless poet..passages which cannot possibly propitiate the people of ‘Down East’.

1909 Relig. Telescope 23 June 23/3
Men who come out of the great West to enlighten the inhabitants of the far-away ‘down East’.

1976 M. LaurenceDown East in C. Martin Local Colour
(1994)
116
Apart from The West and The Coast, our country contained only one other habitable area..and that was Down East. This really meant Ontario. Quebec and the Maritimes existed in geography books but not in our imaginations.

1. In New England and the Maritime Provinces of Canada: in or towards the east along the coast; in or towards more eastern parts. More widely: in or towards the eastern coastal parts of New England (esp. eastern Maine) and the Maritime Provinces.See note at sense A. 1.

1819 Eastern Argus
(Portland, Maine)
11 May
If Maine out counted Massachusetts three to one, would Massachusetts..send her Senators and Representatives ‘down east’ [and some think ‘down east’ the bottomless pit] to Bangor and Castine?

1836 T. C. HaliburtonClockmaker 1st Ser. ii. 12
If I was to tell them in Connecticut, there was such a farm as this away down east here in Nova Scotia, they wouldn't believe me.

1882 Cent. Mag. July 350/1
Antique examples of this curious craft are still to be seen creeping in and out about the little sleepy ports downeast.

1920 S. LewisMain St. xxxv. 416
And the same thing goes for that crowd of crabs and snobs Down East.

1962 Canada Month Feb. 32/2
In Canada, a family can pack up and move from Nova Scotia to B.C. at will. Nobody forces it, though it's of course too bad if it can no longer make a livelihood Down East.

2006 T: N.Y. Times Style Mag. 21 May 144/2
I drive three hours Down East (north on the coast) along the thickly forested coast of northern Maine.

2. In or towards the eastern parts of the United States (esp. New England) or eastern and central parts of Canada (esp. Ontario) from more western parts. Cf. back Eastadv.

1833 Voice of People
(Warren, Pa.)
19 Feb.
‘Davy Crockitt’ has issued his ‘Procklamashun’ stating that if the tariff be not reduced before the fourth of March next, he will go down East and ‘drinck upp thur rivurs, ete upp thur steem-botes’.

1889 Daily Inter Ocean
(Chicago)
25 Aug. 5/1
He..looked as if he wished he could induce the blacksmiths of Chicago to go down East and squeeze out what life yet remains in the New York boom.

1941 Winnipeg Tribune 11 July 9/1
They are talking of getting a divorce and sending us down East to a boarding school.

1991 Carillon
(Steinbach, Manitoba)
27 Nov. 10 c/3
Many friends, neighbors and relatives from down east joined with them and their family to mark their very special occasion.

2002 Edmonton
(Alberta)
Jrnl.
(Nexis)
3 Nov. b8
Kidder plays an undercover cop, Corp. Marina Di Luzio, who is sent from down east to deal with white collar crime in Calgary.